I actually liked Terminator Salvation but I still can't see how Arnie returning to the franchise could help. I mean we all know he cant be the T-800 again so then what?
I suppose an older resistance fightrer who the T-800 was based off could work as long as CHristian Bale returns as John Connor. The only alternative would be, as proposed already, a really old Terminator(120 years was the life span in T2) hanging around either helping what remained of the human resistance in the 22nd century or helping rebuild after the human victory.
EDIT: Actually if you wanted to go REALLY different we could go with a hard sci fi story about all the ex-Terminators living in human society. We know the they could in theory at least adjust according to the learning Terminators of T2. Maybe Terminators need rights too.